The present invention relates generally to integrated circuit technology, including dynamic random access memories (DRAMs) and electrically erasable and programmable read only memories (EEPROMS), and partly, but not by way of limitation, to a floating gate transistor memory that is dynamically electrically alterable and programmable.
Dynamic random access memories (DRAMs) are data storage devices that store data as charge on a storage capacitor. A DRAM typically includes an array of memory cells. Each memory cell includes a storage capacitor and an access transistor for transferring charge to and from the storage capacitor. Each memory cell is addressed by a word line and accessed by a bit line. The word line controls the access transistor such that the access transistor controllably couples and decouples the storage capacitor to and from the bit line for writing and reading data to and from the memory cell.
The storage capacitor must have a capacitance that is large enough to retain a charge sufficient to withstand the effects of parasitic capacitances, noise due to circuit operation, and access transistor reverse-bias junction leakage currents between periodic data refreshes. Such effects can result in erroneous data. Obtaining a large capacitance typically requires a storage capacitor having a large area. However, a major goal in DRAM design is to minimize the area of a DRAM memory cell to allow cells to be more densely packed on an integrated circuit die so that more data can be stored on smaller integrated circuits.
In achieving the goal of increasing DRAM array capacity by increasing cell density, the sufficient capacitance levels of the DRAM storage capacitors must be maintained. A xe2x80x9cstacked storage cellxe2x80x9d design can increase the cell density to some degree. In this technique, two or more capacitor conductive plate layers, such as polycrystalline silicon (polysilicon or poly), are deposited over a memory cell access transistor on a semiconductor wafer. A high dielectric constant is sandwiched between these capacitor plate layers. Such a capacitor structure is known as a stacked capacitor cell (STC) because the storage capacitor plates are stacked on top of the access transistor. However, formation of stacked capacitors typically requires complicated process steps. Stacked capacitors also typically increase topographical features of the integrated circuit die, making subsequent lithography and processing, such as for interconnection formation, more difficult. Alternatively, storage capacitors can be formed in deep trenches in the semiconductor substrate, but such trench storage capacitors also require additional process complexity. There is a need in the art to further increase memory storage density without adding process complexity or additional topography.
Electrically erasable and programmable read only memories (EEPROMs) provide nonvolatile data storage, EEPROM memory cells typically use field-effect transistors (FETs) having an electrically isolated (floating) gate that affects conduction between source and drain regions of the FET. A gate dielectric is interposed between the floating gate and an underlying channel region between source and drain regions. A control gate is provided adjacent to the floating gate, separated therefrom by an intergate dielectric.
In such memory cells, data is represented by charge stored on the polysilicon floating gates, such as by hot electron injection or Fowler-Nordheim tunneling during a write operation. Fowler-Nordheim tunneling is typically used to remove charge from the polysilicon floating gate during an erase operation. However, the relatively large electron affinity of the polysilicon floating gate presents a relatively large tunneling barrier energy at its interface with the underlying gate dielectric. The large tunneling barrier energy provides longer data retention times than realistically needed. For example, a data charge retention time at 85xc2x0 C. is estimated to be in millions of years for some floating gate memory devices. The large tunneling barrier energy also increases the voltages and time needed to store and remove charge to and from the polysilicon floating gate. xe2x80x9cFlashxe2x80x9d EEPROMs, which have an architecture that allows the simultaneous erasure of many floating gate transistor memory cells, require even longer erasure times to accomplish this simultaneous erasure. The large erasure voltages needed can result in hole injection into the gate dielectric. This can cause erratic overerasure, damage to the gate dielectric, and introduction of trapping states in the gate dielectric. The high electric fields that result from the large erasure voltages can also result in reliability problems, leading to device failure. There is a need in the art to obtain floating gate transistors that allow the use of lower programming and erasure voltages and shorter programming and erasure times.
B. Dipert et al., xe2x80x9cFlash Memory Goes Mainstream,xe2x80x9d IEEE Spectrum, pp. 48-52 (Oct. 1993);
S.M. Sze, xe2x80x9cPhysics of Semiconductor Devices,xe2x80x9d John Wiley and Sons, New York (1969), p. 496;
S.R. Pollack et al., xe2x80x9cElectron Transport Through Insulating Thin Films,xe2x80x9d Applied Solid State Science, Vol. 1, Academic Press, New York, (1969), p. 354;
D.A. Baglee, xe2x80x9cCharacteristics and Reliability of 100 xc3x85 Oxides,xe2x80x9d Proc. 22nd Reliability Symposium, (1984), p. 152;
V.M. Bermudez et al. xe2x80x9cThe Growth and Properties of Al and AlN films on GaNxe2x80x9d J. Appl. Physics, Vol. 79, pp. 110-119 (1996).
I. Akasaki et al. xe2x80x9cEffects of AlN Buffer Layer on Crystallographic Structure and On Electrical and Optical Properties of GaN and Ga1xe2x88x92xAlxN Films Grown on Sapphire Substrate by MOVPE,xe2x80x9d J. Of Crystal Growth, Vol. 98, pp. 209-19, North Holland, Amsterdam (1989);
The present invention includes a memory cell that allows the use of lower programming and erasure voltages and shorter programming and erasure times by providing a gallium nitride (GaN) or gallium aluminum nitride (GaAlN) storage electrode for storing charge. According to one aspect of the invention, the storage electrode provides a barrier energy, with an adjacent insulator, of less than approximately 3.3 eV, which provides improved erase times or voltages.
In one embodiment, the memory cell includes a floating gate transistor, having a GaN or GaAlN floating gate. A refresh circuit allows dynamic refreshing of charge stored on the floating gate. The barrier energy can be lowered to a desired value by selecting the appropriate material composition of the floating gate. As a result, lower programming and erasure voltages and shorter programming and erasure times are obtained.
Another aspect of the present invention provides a method of using a floating gate transistor having a reduced barrier energy between a floating gate electrode and an adjacent insulator. Data is stored by changing the charge of the floating gate. Data is refreshed based on a data charge retention time established by the barrier energy between the GaN or GaAlN floating gate and the adjacent insulator. Data is read by detecting a conductance between a source and a drain. The large transconductance gain of the memory cell of the present invention provides a more easily detected signal and reduces the required data storage capacitance value and memory cell size when compared to a conventional dynamic random access memory (DRAM) cell.
The present invention also includes a method of forming a floating gate transistor. Source and dram regions are formed. A gate insulator is formed. A GaN or GaAlN floating gate is formed, such that the floating gate is isolated from conductors and semiconductors. In one embodiment, the floating gate is formed by metal organic chemical vapor deposition (MOCVD). In another embodiment, the floating gate is formed by plasma-enhanced molecular beam epitaxy (PEMBE). The GaN or GaAlN floating gate provides a relatively short data charge retention time, but advantageously provides a shorter write/programming and erase times, making operation of the present memory speed competitive with a DRAM.
The present invention also includes a memory device that is capable of providing short programming and erase times, low programming and erase voltages, and lower electric fields in the memory cell for improved reliability. The memory device includes a plurality of memory cells. Each memory cell includes a transistor. Each transistor includes a source region, a drain region, a channel region between the source and drain regions, and a GaN or GaAlN floating gate that is separated from the channel region by an insulator. The transistor also includes a control gate located adjacent to the floating gate and separated therefrom by an intergate dielectric. According to one aspect of the invention, a refresh circuit is provided. The memory device includes flash electrically erasable and programmable read only memory (EEPROM), dynamic random access memory (DRAM), and dynamically electrically alterable and programmable read only memory (DEAPROM) embodiments.
The memory cell of the present invention provides a reduced barrier energy, large transconductance gain, an easily detected signal and reduces the required data storage capacitance value and memory cell size. The lower barrier energy increases tunneling current and also advantageously reduces the voltage required for writing and erasing the floating gate transistor memory cells. For example, conventional polysilicon floating gate transistors typically require complicated and noisy on-chip charge pump circuits to generate the large erasure voltage, which typically far exceeds other voltages required on the integrated circuit. The present invention allows the use of lower erasure voltages that are more easily provided by simpler on-chip circuits. Reducing the erasure voltage also lowers the electric fields, minimizing reliability problems that can lead to device failure, and better accommodating downward scaling of device dimensions. Alternatively, the thickness of the gate insulator can be increased from the typical thickness of a silicon dioxide gate insulator to improve reliability or simplify processing, since the lower barrier energy allows easier transport of charge across the gate insulator by Fowler-Nordheim tunneling.
According to another aspect of the invention, the shorter retention time of data charges on the GaN or GaAlN floating electrode, resulting from the smaller barrier energy, is accommodated by refreshing the data charges on the floating electrode. By decreasing the data charge retention time and periodically refreshing the data, the write and erase operations can be several orders of magnitude faster such that the present memory is speed competitive with a DRAM. In this respect, the memory operates similar to a memory cell in DRAM but avoids the process complexity, additional space needed, and other limitations of forming stacked or trench DRAM capacitors.
The memory cell of the present invention can be made smaller than a conventional DRAM memory cell. Moreover, because the storage capacitor of the present invention is integrally formed as part of the transistor, rather than requiring complex and costly non-CMOS stacked and trench capacitor process steps, the memory of the present invention should be cheaper to fabricate than DRAM memory cells, and should more easily scale downward as CMOS technology advances.